Any expert got any ideas?
(I used the 5_0 iso from ftp.netbsd)
I have a core dump of the crashed system (ask and I put it somewhere) -
below is a dmesg before the crash.
it very reliable produces "ohci4: 8 scheduling overruns" messages
before crashing.

So I disagree with the flaky memory timing hypothesis and I think it is
a bug in the SATA/kernel area .
(If someone wants access I can open an ssh port to the box and I can
install a serial port redirector to the console plus a CVS checkout )

tj> Asus M4A78-VM
for these Phenom boards, always run a BIOS that was released a couple
months after the CPU you are trying to use. I've had weird
intermittent problems on two different boards both fixed by doing
that. The software burned onto the board is always stale, even if
it's willing to boot the newer CPU.
(problems like, crash at inconsistent places during bootup, or
randomly powerdown for no reason after 1 - 7 days irrespective of
load)
Other people on wikipedia and AOLeet gamerz forums may advise you that
the newest BIOS's on old boards tend to be the work of junior
developers and accumulate regressions, so it may be a balancing act
between rare intermittent problems and subtle regressions.
Another thing about which I've been trying to spread the word for AMD
users:
http://hyvatti.iki.fi/~jaakko/sw/